1. Field of the Invention
The present invention relates to a substrate and an electronic device using the substrate. More particularly, the present invention relates to a substrate on which a capacitor is to be mounted and to an electronic device using the substrate.
2. Description of Related Art
An MPU (Micro Processing Unit) is incorporated in electronic equipment such as a personal computer and a home game machine. A decoupling capacitor is disposed around the MPU. A main role of the decoupling capacitor is to remove noise that is parasitic on a power supply line extending from a power supply to the MPU, and to supply charges to the MPU when a supply voltage is varied.
Recently, with the use of a higher clock frequency in the MPU, the power supply line has become more apt to generate high-frequency noise. For that reason, a low ESL-type multilayer ceramic capacitor that is superior in impedance-frequency characteristic is used as the decoupling capacitor in many cases. Such a low ESL-type multilayer ceramic capacitor is described below with reference to FIGS. 5A and 5B. FIGS. 5A and 5B illustrate one example of low ESL-type feedthrough capacitor. Specifically, FIG. 5A is an external perspective view of a feedthrough capacitor 100, and FIG. 5B is an exploded perspective view of a laminate 102 of the feedthrough capacitor 100.
The feedthrough capacitor 100 is also called a 3-terminal capacitor, and it includes the laminate 102 and external electrodes 104a, 104b, 106a and 106b as shown in FIG. 5A. The laminate 102 is in the shape of a substantially rectangular parallelepiped and is made of stacked ceramic layers. The external electrodes 104a and 104b are formed on mutually opposite side surfaces of the laminate 102. Also, the external electrodes 106a and 106b are formed on the other mutually opposite side surfaces of the laminate 102, which differ from the side surfaces where the external electrodes 104a and 104b are formed.
As shown in FIG. 5B, the laminate 102 includes ceramic layers 108, 110, 112 and 114, capacitor electrodes 116 and 122, and a feedthrough (penetrating) electrode 120. The ceramic layers 108, 110, 112 and 114 are each an insulator layer that is substantially rectangular. The capacitor electrodes 116 and 122 are formed on the ceramic layers 110 and 114, respectively, and have lead electrodes 118a, 118b, 124a and 124b. The lead electrodes 118a and 124a serve to electrically connect the capacitor electrodes 116 and 122 to the external electrode 106a. The lead electrodes 118b and 124b serve to electrically connect the capacitor electrodes 116 and 122 to the external electrode 106b. Further, the feedthrough electrode 120 is formed so as to penetrate the laminate 102. Opposite ends of the feedthrough electrode 120 are electrically connected to the external electrodes 104a and 104b, respectively. The laminate 102 is constructed by stacking the above-described ceramic layers 108, 110, 112 and 114 in this order from the top.
In the above-described feedthrough capacitor 100, the capacitor electrodes 116 and 122 are opposed to the feedthrough electrode 120 with the ceramic layers 110 and 112 interposed respectively therebetween. Therefore, capacity is formed between the capacitor electrode 116 and the feedthrough electrode 120 and between the capacitor electrode 122 and the feedthrough electrode 120. In the above-described feedthrough capacitor 100, since the feedthrough electrode 120 serves also as a power plane, there is essentially no distance from the power plane to a capacity region. As a result, inductance of the feedthrough capacitor 100 is smaller than that of, e.g., a capacitor including a lead line between a power plane and a capacity region.
Hitherto, it has been usual that the feedthrough capacitor 100 is mounted on a substrate shown in FIGS. 6A and 6B. FIGS. 6A and 6B illustrate a known substrate 200 on which the feedthrough capacitor 100 is to be mounted. FIG. 6A is a top plan view of the substrate 200, and FIG. 6B is a sectional structural view of the substrate 200 and the feedthrough capacitor 100. In FIGS. 6A and 6B, the feedthrough capacitor 100 is indicated by dotted lines.
As shown in FIGS. 6A and 6B, the substrate 200 includes a substrate body 202, power planes 204a and 204b, lands 206a and 206b, via-hole conductors 208a and 208b, and a ground plane 210. When the feedthrough capacitor 100 is mounted on the substrate 200, the external electrodes 104a and 104b are electrically connected to the power planes 204a and 204b, respectively. Also, the external electrodes 106a and 106b are electrically connected to the lands 206a and 206b, respectively. The lands 206a and 206b are electrically connected to the ground plane 210, which is embedded in the substrate body 202, through the via-hole conductors 208a and 208b, respectively. The ground plane 210 is grounded.
Further, as shown in FIGS. 7A and 7B, there is known a substrate 300 in which the power planes 204a and 204b are embedded in the substrate body 202. When the feedthrough capacitor 100 is mounted on the substrate 300, the external electrodes 104a and 104b are electrically connected to lands 205a and 205b, respectively. Further, the lands 205a and 205b are electrically connected to the power planes 204a and 204b through via-hole conductors 209-1a, 209-2a and 209-3a and through via-hole conductors 209-1b, 209-2b and 209-3b, respectively.
In either case in which the feedthrough capacitor 100 is mounted on the substrate 200 shown in FIGS. 6A and 6B or on the substrate 300 shown in FIGS. 7A and 7B, a DC current including high-frequency noise is inputted to the external electrode 104a from the power plane 204a. Then, as indicated by solid-line arrows in FIGS. 6B and 7B, the DC current flows through the feedthrough electrode 120 and is outputted to the power plane 204b through the external electrode 104b. On the other hand, as indicated by dotted-line arrows in FIGS. 6B and 7B, the high-frequency noise is outputted to the ground plane 210 through the external electrodes 106a and 106b due to the presence of the capacity formed between the feedthrough electrode 120 and each of the capacitor electrodes 116 and 122. As a result, the high-frequency noise is removed from the DC current.
Lately, miniaturization of semiconductor structures has been progressed in order to employ a higher clock frequency in the MPU and to reduce the size of the MPU. With further miniaturization of semiconductor structures, a threshold voltage is reduced, and an operating voltage of the MPU is reduced correspondingly. On the other hand, the design rule of 90 nm or less tends to increase a leakage current generated inside a semiconductor and to increase power consumption of the MPU. Accordingly, a larger current has to be supplied from a power supply to an MPU using highly miniaturized semiconductors. This results in a larger current flow in the feedthrough electrode 120 of the feedthrough capacitor 100 which is disposed around the MPU.
However, when a larger current flows in the feedthrough electrode 120, the feedthrough capacitor 100 generates heat due to a residual resistance (i.e., an equivalent series resistance (ESR)) of the feedthrough electrode 120. In order to suppress the generation of heat, for example, Japanese Unexamined Patent Application Publication No. 6-349678 proposes an electronic device, and Japanese Unexamined Patent Application Publication No. 2003-282347 proposes a capacitor mounting structure. The proposed electronic device and capacitor mounting structure will be described below with reference to the drawings. FIGS. 8 and 9 are plan views each showing an electronic device described in Japanese Unexamined Patent Application Publication No. 6-349678. FIG. 10 is a sectional structural view of an electronic device in which the capacitor mounting structure according to Japanese Unexamined Patent Application Publication No. 2003-282347 is employed.
In the electronic devices shown in FIGS. 8, 9 and 10, a large current is prevented from flowing in the feedthrough electrode 120 by forming an additional current path in parallel to a current path formed by the feedthrough electrode 120. More specifically, in the electronic devices shown in FIGS. 8 and 9, the lands 205a and 205b, which are electrically connected to the power planes 204a and 204b (not shown in FIGS. 8 and 9), respectively, are electrically connected to each other through a wiring conductor (short-circuit electrode) 207. Also, in the electronic device shown in FIG. 10, the external electrodes 104a and 104b are electrically connected to the lands 205a and 205b, respectively, which are formed on the substrate body 202. The lands 205a and 205b are electrically connected to the power planes 204a and 204b, which are formed inside the substrate body 202, through the via-hole conductors 209-1a, 209-2a and 209-3a and the via-hole conductors 209-1b, 209-2b and 209-3b (only the via-hole conductors 209-1a and 209-1b being shown in FIG. 10), respectively. The power plane 204a and the power plane 204b are electrically connected to each other also through a wiring conductor (short-circuit electrode) 207. According to each of the electronic devices constructed as described above, a current inputted from the power plane 204a is divided and flows along the current path formed by the feedthrough electrode 120 and the additional current path formed by the wiring conductor 207 (see, e.g., arrows in FIG. 10). As a result, a large current is prevented from flowing in the feedthrough electrode 120.
However, the known electronic devices have the following problems. In the electronic device shown in FIG. 8, the wiring conductor 207 is formed in a linear shape. Inductance of the wiring conductor 207 in the linear form is relatively small. Therefore, impedance of the current path formed by the linear wiring conductor 207 is also relatively small. Hence, a part of the high-frequency noise is inputted to the current path formed by the wiring conductor 207. Consequently, the electronic device shown in FIG. 8 has the problem that the high-frequency noise cannot be sufficiently removed from the DC current.
In the electronic device shown in FIG. 9, the wiring conductor 207 is bent so as to bypass the feedthrough capacitor 100. The bent wiring conductor 207 has larger impedance than the linear wiring conductor 207 shown in FIG. 8. Accordingly, the electronic device shown in FIG. 9 can remove the high-frequency noise from the DC current to a larger extent than the electronic device shown in FIG. 8. However, the electronic device shown in FIG. 9 needs to have a space for the bent wiring conductor 207, whereby the size of the electronic device is increased.
In the electronic device shown in FIG. 10, the current path formed by the feedthrough electrode 120 includes the via-hole conductors 209-1a, 209-2a, 209-3a, 209-1b, 209-2b and 209-3b. Each of the via-hole conductors 209-1a, 209-2a, 209-3a, 209-1b, 209-2b and 209-3b has relatively large inductance. Accordingly, impedance of the current path formed by the feedthrough electrode 120 is also relatively large. Hence, a part of the high-frequency noise inputted from the power plane 204a is inputted to the current path formed by the wiring conductor 207. Consequently, the electronic device shown in FIG. 10 has the problem that the high-frequency noise cannot be sufficiently removed from the DC current.